In order to provide a more logical basis for the selection of microbial agents for the potential control of Biomphlaria spp. and Bulinus, spp., the molluscan intermediate hosts for the human pathogens Schistosoma mansoni and S. haematobium, respectively, the cellular internal defense mechanisms of these gastropods are being studied. Specifically, we are examining (1) whether chemotaxis occurs between hemolymph cells of these molluscs and bacterial cells that are known to be phagocytized; (2) what the basis is for the recognition of self and nonself; (3) what enzymes occur in the phagocytes of the species of molluscs being studied as well as in the serum, and the role of such enzymes in the degradation of certain invading microorganisms.